Biomechanics of Human Actuators

2.2 Biomechanics of Human

Figure Human’s movement of freedom of our body. How and power are performed running, standing up and focused on actuation of joint and ankle as seen in [30]. There are many deg a waste to actuate all of the [30], the flexionextension be actuated. man ure 2.1: Reference plane of human body.[35] ent and motion are very complex due to the vas However, most of our daily life motions that r ed in the Sagittal plane. [34] These motions d more. This is the reason why many of the oint in Sagittal plane particularly in flexionext degrees of freedom for human’s lower limb. How them as some of them do not require high pow n of hip, knee and ankle requires the most pow vast number of degree t require large torque ons include walking, the previous studies xtension of hip, knee However, it would be power. According to power which needs to In conclusion, actuations should be provided to the flexionextension of hip, knee and ankle whereas the other un-actuated joints are left to rotate freely to minimize the hindrance of the user’s movements.

2.3 Actuators

Actuators are the device that provide motion for the exoskeleton and hence support the wearer’s limb motion. It takes the command from the controller and provides motion according to the requirement at a certain instant. Electrical Actuator is one of the most popular choices due to its clean and silent operation as well as lower power consumption than others [5]. Some of the electrical actuators used by previous studies include DC Servomotor, Linear Actuator and Series Elastic Actuator. DC Servomotor is a popular choice in providing actuation for robots as well as exoskeletons. It is essentially a motor with built-in position feedback. One of the reasons it is often used as an actuator is because of its simplicity and convenience to apply. [21] DC Servomotor can be easily controlled by controller which sends signal to it. Unlike any other actuator, the direction, speed and even position of the motor can be easily controlled by electronics. Moreover, DC servomotor provides precise positioning because of its built- in position feedback which gives an accurate control of its motion and position. . It provides clean and silent actuation of exoskeleton compared to the other which makes it suitable for application in any environment, indoor or outdoor. Apart from that, it is relatively compact compared to other electrical actuators is also one of the reason why it is such popular choice. [21] This is why it is normally positioned on the exoskeleton’s joint that need to be actuated as seen in [18, 19, 21, 27]. However, there are some studies that position it away from the joint and transmit power through a drive system like chain or cable. [20, 23] Despite the compact size, a DC Servomotor with high torque and small size is not very easy to find. That is why Servomotors are normally coupled with suitable reducers to provide high torque and fast response motion in a small package. DC Servomotors are chosen to be used by previous studies are also because of its high energy efficiency. [21] There are studies that use linear actuator as their actuation system for their exoskeleton. [22, 23] Linear actuator consists of an electric motor and a leadscrew which translate the rotational motion provided by the motor into linear motion. It is chosen by studies in [22, 23] because of its high load capacity which means it can provide high force output. Apart from that, reasonable price is also one of the advantages of the linear actuators. However, there are some drawbacks of this type of actuator. One of it being that it is normally large and bulky because it needs to house the components like leadscrew, motor, gearing, bearing and other components. Besides that, the moving parts of the actuator are prone to wear and tear due to frictions. The linear actuator also has a limited range of motion as the leadscrew can only extend for a limited length only. Figure 2.2: Linear series elastic actuator[25] Linear Series Elastic Actuator is also a type of actuator used to provide motion for the exoskeleton. This actuator utilizes an electric motor coupled with lead ball screw and spring pack to provide linear motion as shown in Figure 2.2. It has a spring pack in series with the output of the motor. Some studies chose this type of actuator because it provides a better way to implement light and inexpensive force control. [25] Apart from that, it is said to be shock tolerant, low impedance and provides energy storage due to the spring pack incorporated in the design. [10] However, the size of Series Elastic Actuator is usually larger than other alternatives because it the components it needs. It is very hard to fit an